1. Field of the Invention
The present invention relates to a magnetic material for microwave and millimeter wave frequencies and, more particularly, to a magnetic composition for high frequency circuit elements designed for use in the microwave and millimeter wave frequency ranges.
2. Description of the Prior Art
Heretofore Mn-Mg ferrites, Ni-Zn ferrites, lithium ferrites and YIG ferrites have been used as magnetic materials for high frequencies, because as they possess a high saturation magnetization (4.pi.Ms) ranging from 500 to 4000 gauss. In particular, the YIG ferrites are used in the production of highly stable circuit elements such as isolators and circulators.
U.S. Pat. No. 3,132,105 discloses that the saturation magnetization (4.pi.ms) and the temperature coefficient 4.pi.ms of a YIG ferrite may be controlled by replacing some part of the ingredients with Gd and Al. Such a modified composition makes it possible to select a magnetic material with an optimum value for the saturation magnetization moot pertinent to the operating frequencies of the circuit elements to be produced. Also, a combined use of a YIG ferrite with a permanent magnet makes it possible to compensate for the temperature characteristics of the magnet.
U.S. Pat. No. 3,419,496 discloses that the incorporation of MnO.sub.2 into Y.sub.3 Fe.sub.5 O.sub.12 makes it possible to produce a magnetic material with a volume specific resistance (.rho.) of 4.9.times.10.sup.12 .OMEGA..multidot.cm which is greater than that of the basic Y.sub.3 Fe.sub.5 O.sub.12 composition, i.e., 7.0.times.10.sup.8 .OMEGA..multidot.cm. Japanese patent publication No. 60-55970 discloses a ferrite material of a YIG system consisting essentially of 38.63 to 39.45 mol% of Y.sub.2 O.sub.3 and 61.37 to 60.55 mol% of Fe.sub.2 O.sub.3. That material possesses a ferromagnetic resonance line width (.DELTA.H) of 16 Oersted (Oe) at the minimum.
However, the YIG ferrites of the prior art have a disadvantage in that a very small deviation from the composition results in considerable increases in the ferromagnetic resonance line width (.DELTA.H) and in the dielectric loss tangent (tan .delta.e). This makes it impossible to put the prior art YIG ferrites into practical applications. Also, the greater the remanent magnetic flux density (Br) the greater is the dielectric loss tangent. As a result the ferrites cannot be applied to production of elements for phase converters which are required to have a high remanent magnetic flux density.